1. Field of the Invention
This invention relates to flexible shafts and coupling designs that provide a greater control and can be adapted to an improved flexible shaft for the transmission of rotary motion and power around, over or under obstacles, i.e. reaming the medullary canal of bones, vertebral body replacement implant, as well as forming a segment of the spinal column of anthropomorphic dummies.
2. Brief Description of the Prior Art
Flexible Shafts
Flexible shafts and couplings are used to transmit rotary power between a power source and a driven part in a curvilinear manner. These shafts are used when there is little or no accurate alignment between the power source and the driven part; when the path between the power source and the driven part is blocked or is in an environment or position which would not allow the power source; for the connection or driving of components which have relative motions; and to dampen and absorb vibration both from the drive unit and the driven tool.
A flexible shaft generally consists of rotating shaft with end fittings for attachment to mating parts, typically a power source and the driven part, as depicted in FIG. 3 of U.S. Pat. No. 4,646,738, Suhner catalog at page 6, and the S.S. White Technologies Inc. catalog, page 4, (1994). A protective outer casing can be used to protect the shaft when necessary. Flexible shafts are used in numerous applications anywhere the transmission of rotary power is necessary and a straight unobstructed path is unavailable, as depicted in the S.S. White Technologies Inc. catalog, page 5 and Suhner at page 6. Flexible shafts have been used in children""s toys to aerospace applications. Examples of the usage of flexible shafts have been presented in the articles xe2x80x9cNew Twists for Flexible Shaftsxe2x80x9d (Machine Design, Sep. 7, 1989), in particular pages as illustrated on pages 145 and 146, and xe2x80x9cFlexible Shafts Make Obstacles Disappearxe2x80x9d (Power Transmission Design, July, 1993), in particular FIG. 1. One example cited was a safety valve, located 30 ft. off the ground and not readily accessible, that had to be operated on a daily basis to remain operable, but was not exercised as regularly as required due to the difficulty in reaching it. With the installation of a flexible shaft from the valve to floor level, personnel were able to operate the valve regularly and verify its proper function. Flexible shafts are used on aircraft to raise and lower wing flaps, slats, and leading and trailing edges. Stainless steel flexible shafts allow surgeons greater maneuverability with bone cutting and shaping tools. Flexible shafts are also used extensively to compensate for less than perfect alignment between a driver and a driven component. The limitation for the use of flexible shafts are limitless and is only limited by the torque capabilities of the shaft.
Heretofore, flexible shafts and couplings available for power transmission consisted of single or multiple wires wound over a central drive core or a hollow core, as illustrated in U.S. Pat. No. 5,108,411, FIG. 2, and as depicted in the Suhner publication, pages 15 and 16. The number of wires per layer and the number of layers will vary according to the application and requirements for either unidirectional or bi-directional torque power transmission. Typically wire wound flexible shafts are designed and manufactured to be operated in only one direction of rotation; either clockwise or counter clockwise, when viewed from the driving end. They are designed to maximize the torque carrying capabilities for the direction of rotation for which they were designed. The performance of a unidirectional shaft operated in the reverse direction is significantly less than the intended performance levels.
A specific application of flexible shafts is with flexible medullary canal reamers. Medullary canal reamers are used to enlarge the medullary canal of bones in preparation for the insertion of prosthetic components, such as a total hip prosthesis, the insertion of fracture reduction and fixation devices, such as intramedullary nails, performing an intramedullary osteotomy, the insertion of a plug to preclude bone cement from migrating while in its viscous state, stimulating bone growth, and for other purposes. Since the medullary canal is irregular in internal diameter and configuration from end to end it is preferred by the surgeon to enlarge the medullary canal to a more uniform diameter or to a diameter that will allow passage or insertion of the intended device. Because the shafts of long bones are bent or curved along their longitudinal axes, flexible shafts that can bend to follow this naturally curved path while transmitting the necessary torque required to cut the bone are necessary.
Should a straight, rigid, or inflexible shaft be used in the reaming process to enlarge the canal, there is considerable likelihood that the reamer will not follow the natural curvature of the bone, will not remove the desired amount of bone and will not produce a uniform internal diameter. In addition, should a straight, rigid reamer be used, there is a high degree of likelihood that the reamer will jam, cause excessive bone removal or penetrate the outer integrity of the bone. For this reason, medullary canals are almost always prepared with reamers having a flexible shaft. Flexible medullary reamers are of such design that utilizes a central bore intended to receive a long, small diameter guide rod or wire that is initially inserted into the medullary canal. The guide wire or rod establishes a track for the advancing reamer. However, the use of a flexible reamer does not preclude the problem of jamming or reamer stoppage when the cutting head of the reamer gets caught by the bony structure and does not turn. A jammed cutting head may be extremely difficult, if not impossible to dislodge or remove without further violation of the involved bone or breakage of the reaming device. The preferred method to dislodge the reamer would be to reverse the reamer. However, the design of the most widely used devices prevent the reversal of the reamer without destruction of the flexible shaft.
Heretofore, the flexible medullary shaft reamers available to the orthopedic surgeon are of three types: (i) a shaft with a plurality of parallel flexible elements or rods joined together at opposite ends by means of a welded of soldered connection, (ii) a shaft comprised of a spiral or helically wound metal wire(s) or strip(s), and (iii) a shaft comprised of a series of inter-engaged links, assembled over a guide rod.
The first distinct type of flexible medullary reamer (i) embodies a plurality of parallel, flexible elements joined together at opposite ends. A disadvantage of this shaft occurs during usage as the reamer rotates causing the elements to become twisted and thereby to become more rigid and reduce the shaft""s flexibility. Another disadvantage of said reamer is the shaft""s tendency, as it rotates but is not yet fully within the confines of the medullary canal, to tear tissue from underlying structures as the individual elements are torsionally loaded and unloaded, thereby enlarging and contracting the spaces between the individual wires to trap uninvolved tissue and tearing them free.
Another disadvantage of said flexible reamer occurs during insertion of the reamer over the guide rod. The central bore is intended to receive the small diameter guide rod. Except at its respective ends, this reamer lacks a well defined and bordered central bore. Therefore it is difficult to prevent the guide rod from exiting the reamer in the area of the free standing elements during the insertion of the guide wire. A further disadvantage of this flexible shaft is the inefficient transfer of energy from the power source to the cutting head that is caused by the twisting and wrapping together of the individual elements as the reamer is rotated. Another disadvantage of this type of reamer is that it is extremely noisy during operation due to the multiple elements hitting one another during the rotation.
The second distinct type of flexible medullary reamer (ii) consists of spiral or helically wound metal wires or strips. This is the most widely used flexible shaft for intramedullary reaming. The major disadvantage of this reamer design is that it can only be operated in the forward mode of operation. If the cutter becomes jammed and the surgeon reverses the reamer to dislodge the cutter or to facilitate removal, the shaft unwinds, thus rendering the reamer permanently deformed, unusable, and unrepairable. A further disadvantage of this medullary reamer is that the tensional load to which it is subjected when in use results in poor power transfer and varying degrees of distortion of said shaft. If the power source providing the rotational energy to the reamer is great enough, the coils can tighten sufficiently to adversely affect the structural integrity of the shaft and cause the shaft to permanently deform into a helical shape. A further disadvantage of this type of reamer is the inability to clean the shaft and the cavities within the helically wound strips of surgical debris after the operation for the prevention of cross contamination between patients. If infectious blood or body fluids infiltrates the mechanism of the device, it is extremely difficult to remove and clean.
The third distinct type of flexible shaft (iii) consists of a series of inter-engaged links assembled over a guide wire. A distinct disadvantage of this design is during usage and inter-changing the cutting head. The current usage of this design dictates that the links are held together by a longitudinal guide wire over which the linkages are assembled. In order to change the cutting head, a flexible tube must be inserted through the central bore of the linkages, and the assembled links must be taken off the centralizing guide wire. In the process linkages frequently become unassembled and require the surgeon to reassemble the linkages.
U.S. Pat. No. 5,488,761 to Leone, shows prior art spiral wound flexible shafts using a single shaft and a pair of reverse wound shafts. The patent also discloses materials of construction or the shaft and a mechanism for cleaning the slot, after it is cutting. Alternate cutting technologies are also disclosed.
The prior art is depicted in Matthews, U.S. Pat. No. 4,706,659 which show two modifications of prior art devices, in FIGS. 1 and 2. The device of Matthews is loosely related to the present invention in that it is a mechanism for providing a flexible connecting shaft for an intramedullary reamer. While the proposed solution to the problem is different from that of the present invention, the patent discloses the importance of a flexible connection and discloses reamer structures. The disclosure of Matthews 4,706,659 is incorporated by reference herein, as though recited in full.
U.S. Pat. No. 4,751,922 (DiPietropolo) also shows the importance of flexible medullary reamers and explains some of the prior art problems. The patent also discloses the use of a hollow core 2, for receiving a guide pin. U.S. Pat. No. 5,122,134 (Borzone et al) is incorporated by reference as though recited in full and is noted to disclose in FIG. 5, the use of a guide pin 55.
FIG. 1 of Zublin, 2,515,365 illustrates a flexible drill pipe for use in the drilling of well bores. Additional Zublin patents include U.S. Pat. Nos. 2,515,366, 2,382,933, 2,336,338 and 2,344,277. The drill pipe is a helically slotted flexible drill pipe having a slot varying from {fraction (3/32)}of an inch (0.0938xe2x80x3) to {fraction (5/32)} of an inch (0.1563xe2x80x3) in width and having a pitch of the spiral of about nine inches for a four and one-half inch diameter drill pipe (helix angle of 32.48 deg). Zublin indicates that the described flexible resilient drill pipe has the capacity to bend into a curve of an eighteen foot diameter utilizing a repeating xe2x80x9cdovetailxe2x80x9d pattern of over six cycles per revolution, for use with four and one half inch diameter drill pipe. In the instant invention, it has been found that shafts of one inch or less require the use of a helix angle of approximately one half that described by Zublin and that the number of repeating cycles of the interlocking pattern is less than the shown six cycles per revolution. For the smallest of flexible shafts describe, the use of about two pattern repetitions (cycles) per spiral revolution is more appropriate.
Spinal Element
The disclosed technology can also provide a tubular structure with certain stiffness characteristics that have the controllability to be varied dependent upon the application. An example of this would be in automotive and aircraft crash test evaluation and aircraft ejection seat response. In applications where the test is aimed at determine a human response, it is desirable to provide a spinal element to be inserted into a test manikin which has the biofidele response of the human spinal column or segment.
Various spine mechanisms have been developed for anthropomorphic test devices (ATD). The devices can be divided into vertically stacked vertebral simulating members, U.S. Pat. Nos. 3,754,338, 3,753,301, 3,753,302, 3,762,070, and 3,962,801 or a single unit member typically composed of a solid rubber beam, U.S. Pat. No. 5,152,692.
Richards (5,152,692) discloses a biofidelic manikin neck comprising a butyl rubber beam inserted between an aluminum base and an aluminum top, with a stainless steel cable assembly in the middle of the beam connecting the top to the base. In an embodiment of the invention, a pivot joint is bolted to a lug in the neck top, with the joint being attached to a head mounting plate to provide head nodding action. An outer cylinder, comprised of a thick sheet of butyl rubber attached vertically around the head mounting plate and the neck base, allows the head moment to respond to the angle between the head and the torso. A second embodiment of the invention uses a torsion release swivel joint instead of a pivot joint with additional strips of butyl rubber to stiffen the neck response. A third preferred embodiment uses a rectangular inner beam, and a roughly oval-shaped outer support structure.
Vertebral Body Replacement Implant
In situations when a vertebra is broken, crushed or diseased, it is frequently necessary to ablate the body of the crushed or diseased vertebra. In order, however to prevent the spinal column from collapsing with damage to the spinal cord running in the vertebral foramen forward of the vertebral body, it is necessary to employ a spacer. The spacer is braced vertically between the bodies of the adjacent vertebrae, maintaining them at the desired separation distance. A substitute vertebra with biofidelic properties would provide the optimum replacement.
Various implants have been developed to address structural failure of various parts of the spinal column. The prior art with respect to spinal column implants falls into two general categories: intervertebral disc prostheses, and rigid vertebral body prostheses.
Vertebral body prostheses have been disclosed in U.S. Pat. Nos. such as 3,426,364, 4,401,112, 4,554,914, 4,599,086, 4,932,975, and 5,571,192. The referenced patents typically are composed of a rigid, height adjustable device and are typically a threaded cylinder or turnbuckle mechanism with anchoring plates. Another type of replacement device is composed of individual elements that are sized and adapted to be fitted together to provide support to the adjacent vertebra. This type of device has been described in U.S. Pat. Nos. 5,147,404 and 5,192,327.
The devices presented in the foregoing patents are intended for situations where it is necessary to remove a vertebral body. That, in turn, requires the resection of adjacent intervertebral discs. A problem common to all of such prior devices is that while they adequately provide the structure of the removed vertebral body, they fail to provide the flexibility of the removed intervertebral discs.
The disclosed invention provides a cylindrical device, containing a helical pattern than, when adapted to the end use, provides a flexible column that can be used in a variety of applications. When adapted for use with as flexible shaft, the device will flex, bend or curve to follow the natural intramedullary canal of the bone while transmitting reaming torque. When adapted for use as a vertebral body replacement implant, the device provides axial, bending, and torsional stiffness can provide the mechanical characteristics of the vertebral body and disc specified for replacement. The axial, bending and torsional stiffness can be modified for use as an anthropomorphic spine unit that faithfully reproduces human-like responses in ejection seat and vehicle crash tests, enabling researchers to identify and eliminate the cause of spinal injuries.
These and other features, advantages and aspects of the present invention will be better understood with reference to the following detailed description of the preferred embodiments when read in conjunction with the appended drawing figures.
The present invention overcomes the deficiencies and problems evident in the prior art as described herein above by combining the following features into an integral, longitudinally flexible and torsionally inflexible shaft. The disclosed flexible shaft provides for the transmission of rotary power from a drive power unit to a driven unit. The driven unit can be a drill bit, a surgical reamer, a pump, or any similar device. The flexible shaft is an elongated tubular member of substantial wall thickness. The diameter of the shaft is preferably in the range from about 0.15 inch to about 4.00 inch. The ratio of the diameter of the inside diameter of the shaft to the outside diameter of the shaft is advantageously in the range from about 1:1.2 to about 1:3, and preferably is in the range from about 1:1.3 to about 1:4. The thinner the wall of the shaft, the more critical is the configuration of the slot.
A slot of substantial length and width extends in a generally helical path, either continuously or intermittently, around and along the tubular member. Advantageously, the slot is cut at an angle normal to the shaft using a computer controlled cutting technique such as laser cutting, water jet cutting, milling or other means. The slot follows a serpentine path along the helical path generally around and along the tubular member, corresponding generally to the form of a signal wave on a carrier wave, that is, an amplitude modulated carrier wave. Additionally, this slot may be cut at an angle to the normal so as to provide an undercut slot, preferably the angle is in the range from about 30 to about 45 degrees from the normal.
A plurality of slots, can also be employed thereby increasing the flexibility of the shaft, relative to a shaft having a single slot of identical pattern. The serpentine path forms a plurality of teeth and complimentary recesses on opposite sides of the slot. The slot has sufficient width to form an unbound joint permitting limited movement in any direction between the teeth and the recesses, thereby providing limited flexibility in all directions upon application of tensile, compressive, and/or torsion forces to said shaft.
Different degrees of flexibility along the length of said shaft can be achieved by having the pitch of the helical slot vary along the length of the shaft. The varied flexibility corresponds to the variation in the pitch of the helical slot. The helical path can have a helix angle in the range of about 10 degrees to about 45 degrees, and the helix angle can be varied along the length of the shaft to produce correspondingly varied flexibility. Alternatively, the width of the helical slot can vary along the length of the shaft to provide the varied flexibility. Advantageously, the width of the slot is preferably in the range from about 0.005 inch to 0.075 inch. Preferably the width of the slot is in the range from about 0.01 to about 0.05 inch. The rigidity of the flexible shaft can be achieved through the design of the slot pattern, thereby enabling the use of thinner walls than would otherwise be require to produce equivalent rigidity. In a preferred embodiment, the ratio of the amplitude of the serpentine path to the pitch of the slot is in the range from greater than 0.1 to about 0.5.
The slot can be filled with a resilient material, partially or entirely along the path of the slot. The resilient material can be an elastomer, such as urethane or a silicone compound, which can be of sufficient thickness to fill the slot and to encapsulate the entire shaft thus forming an elastomer enclosed member.
In a preferred embodiment the driven unit is a medullary canal reamer, for use in reaming the medullary canal of bones. In this application, the foregoing slot patterns and shaft dimensions, are particularly critical.
Preferably, the flexible shaft, is formed by laser cutting an elongated tubular member of substantial wall thickness, to form the slot around and along the tubular member. The serpentine path can form of a generally sinusoidal wave superimposed on a helical wave. Preferably, the sinusoidal wave forms dovetail-like teeth, which have a narrow base region and an anterior region which is wider than the base region. Thus, adjacent teeth interlock.
Anthropomorphic Spine Unit
This embodiment provides an apparatus for protecting occupants of vehicles in crashes or in sudden positive or negative accelerations. More specifically, it relates to a spinal unit for use in anthropomorphic dummies or manikins intended to simulate human occupants in testing for the effectiveness of protective equipment.
The development of test dummies demonstrates that they have been designed primarily for applications in automobiles. One of the main concerns in automobile crashes is the response of the torso to impacts from the forward and lateral directions. Ejection seat manikins, however, are subjected to sudden vertical accelerations as well as to horizontal acceleration. The manikins are much more sophisticated representations of the human body and have been developed specifically for ejection seat testing by the Armed Services. Although the accuracy of the test results are dependent on the biofidelity of the ATD to provide the response that of a human, the current spinal units incorporated into either automobile crash dummies or aircraft manikins do not faithfully model the motion of the human spine. Therefore a spinal unit incorporated in an ATD that has the mechanical properties of the human spine segment (lumbar, thoracic or cervical) will provide improved test results and evaluation of restraint systems.